Lassaigne's Test: N, S & Halogens
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flagWhat you'll discover
- arrow_forwardExplain why sodium fusion is needed before testing organic compounds for N, S and halogens
- arrow_forwardDescribe the preparation of the sodium extract (fusion filtrate)
- arrow_forwardPerform and interpret the Prussian blue test for nitrogen
- arrow_forwardPerform the nitroprusside test for sulphur and the AgNO₃ test for halogens
- arrow_forwardExplain why the extract is boiled with HNO₃ before the halogen test
Why fusion first: covalent to ionic
Nitrogen, sulphur and halogens in organic compounds are bound covalently — locked inside molecules where ionic test reagents simply cannot see them. Add AgNO₃ to chloroform and nothing happens, despite three chlorines per molecule. Lassaigne's insight (1843): destroy the molecule and capture the elements as simple ions. A pea-sized piece of sodium is heated to red heat with the compound in a small ignition tube; the violent reduction converts nitrogen to NaCN, sulphur to Na₂S, halogens to NaX. If both N and S are present, NaSCN (sodium thiocyanate) can form too.
The red-hot tube is then plunged into distilled water in a china dish — it shatters, excess sodium is destroyed, and the ionic products dissolve. Boiling and filtering gives the clear sodium extract (fusion filtrate), the solution on which every subsequent test is run. The fusion must reach true red heat: incomplete fusion is the single commonest cause of false negatives.
Prussian blue and the violet of sulphur
Nitrogen test: to a portion of the extract add freshly prepared FeSO₄ solution and boil — Fe²⁺ combines with cyanide to form sodium ferrocyanide: 6NaCN + FeSO₄ → Na₄[Fe(CN)₆] + Na₂SO₄. Cool, add a little FeCl₃ and acidify with concentrated HCl: ferric ions meet ferrocyanide and the deep Prussian blue of Fe₄[Fe(CN)₆]₃ appears. The acid dissolves the dirty iron hydroxides so the blue shows cleanly.
Sulphur test: to a fresh portion add a few drops of sodium nitroprusside solution. Sulphide ion converts the reagent into the intensely violet complex Na₄[Fe(CN)₅NOS] — instant and unmistakable. (A backup test: acidify with acetic acid and add lead acetate — a black precipitate of PbS also confirms sulphur.) If nitrogen and sulphur occur together, the extract may contain thiocyanate, which gives a blood-red colour with Fe³⁺ instead of Prussian blue — a famous interpretive trap.
Halogens — after destroying the interference
The halogen test looks easy: add AgNO₃, look for the precipitate — white AgCl (soluble in NH₄OH) for chloride, pale-yellow AgBr (sparingly soluble) for bromide, yellow AgI (insoluble) for iodide. But if the compound also contained N or S, the extract holds CN⁻ and S²⁻, and both form silver precipitates of their own — false positives waiting to happen.
The fix: boil the extract with dilute HNO₃ first. Boiling expels the interfering ions as volatile gases: CN⁻ leaves as HCN, S²⁻ as H₂S. Only then is AgNO₃ added, and any precipitate that remains is a genuine halide. The colour plus the ammonia-solubility behaviour identifies which halogen. This boil-first step is precisely the kind of "why do we do this?" detail the practical viva loves — and in the simulation, the test result will warn you about interference when N or S is present.